1. Field of the Invention
The present invention relates to the treatment of subterranean hydrocarbon-containing formations with viscous aqueous polymer solutions, and particularly, to the use of such polymer solutions for enhancing the production of hydrocarbons from subterranean hydrocarbon-containing formations penetrated by one or more well bores.
2. Prior Art
Viscous aqueous solutions containing organic polymers previously have been utilized for carrying out a variety of treatments in subterranean hydrocarbon-containing formations to increase the production of hydrocarbons therefrom. For example, in the production of oil and gas wells which also produce large volumes of salt water, the cost of producing the salt water, separating it from produced oil and gas and disposing of the salt water represents an economic loss in the operation of the wells. In order to reduce the production of salt water from such wells, viscous aqueous polymer solutions have heretofore been utilized to preferentially reduce water production from portions of the producing subterranean formations. The high molecular weight polymers or copolymers in the aqueous polymer solution plug the pores of the portions of the formation producing water and thereby substantially reduce the permeability of such portions whereby water production is also substantially reduced.
Acid treating or acidizing of porous subterranean formations penetrated by a well bore has been widely employed for increasing the production of fluids, such as crude oil, natural gas and the like from said formations. The usual technique of acidizing a formation comprises introducing a non-oxidizing acid into the well under sufficient pressure to force the acid out into the formation where it reacts with the acid-soluble components of the formation.
During the acid treating operation, passageways for fluid flow are created in the formation, or existing passageways therein are enlarged, thus stimulating the production of fluids from the formation. This action of the acid on the formation is often called etching.
Hydraulic fracturing is also commonly employed to increase the production of fluids from subterranean formations. Hydraulic fracturing comprises the injection of a suitable fracturing fluid down a well penetrating a formation and into said formation under sufficient pressure to overcome the pressure exerted by the overburden. This results in creating a crack or fracture in the formation to provide a passageway which facilitates flow of fluids through the formation and into the well. Combination fracture-acidizing processes are well known in the art.
One of the problems commonly encountered in acidizing operations is insufficient penetration of the formation by the acid. It is desirable that good penetration be obtained in order to realize maximum benefits from the operation. Too often the acid is essentially completely spent in the area immediately adjacent to and surrounding the well bore. The severity of the problem increases as the well temperature increases because acid reactivity with the formation increases with increasing temperatures, such as are encountered in deeper wells.
Poor penetration can be also either caused or aggravated by fluid loss to the more porous zones of the formation where low permeability is not a problem. Poor penetration can also be either caused or aggravated by leak-off at the fracture faces in combination fracturing-acidizing operations. Either said fluid loss or said leak-off can frequently worsen the situation by leaving the tight (low permeability) zones of the formation unchanged by merely penetrating the already high permeability zones.
Higher viscosities are also advantageous in combination fracturing-acidizing operations in that the more viscous acidic solutions produce wider and longer fractures. More viscous acid solutions are also more effective in carrying propping agents into the formation when propping agents are used.
Viscous aqueous fluids containing organic polymers also are used as completion fluids when conducting downhole operations after initial drilling or while waiting to put the well into production. Completion fluids are used to control formation fluid pressure before the well is put on production and while mechanical work is being performed on the formation, for example, perforating, underreaming, milling or the like. The loss of completion fluids to the formation is undesirable. The fluid loss generally can be reduced by increasing the viscosity of the completion fluid.
It would be desirable to provide a method by which an aqueous fluid can be viscosified with a polymer or mixture of polymers which minimizes the quantities of polymer required to recover hydrocarbons from subterranean hydrocarbon-containing formations penetrated by one or more well bores.